Scattering takes place when parallel streams of photons (waves of light) meet an obstacle and deviate in different directions.
In this resource, scattering is used to refer to the different forms of deviation associated with diffusion, dispersion, interference, reflection and refraction as well as by the composition and surface properties of different media.
Scattering does not take place:
- When parallel rays of light reflect off a smooth flat surface such as a mirror and so produces a distortion-free reflection.
- When parallel rays of light reflect off a smooth convex surface though the reflection does appear magnified.
- When parallel rays of light reflect off a smooth concave surface, though in most cases the reflection does appear in miniature and inverted.
- When parallel rays of light are transmitted through translucent media containing dissolved materials such as dyes.
About regular scattering
- When parallel rays of light reflect off a curved surface such as a convex or concave mirror scattering produces a magnified or miniature distortion-free reflection.
- Deflection of parallel rays of incident light passing through objects such as raindrops and prisms, which have smooth surfaces and transparent interiors, causes regular scattering.
- Regular scattering is often accompanied by chromatic dispersion that causes the separation of white light into its component spectral colours.
- Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength produce a rainbow effect.
- At a microscopic level all forms of scattering obey the laws of reflection and refraction (Snell’s law).
Let’s consider two cases of regular scattering in more detail:
- When parallel rays of light of a single wavelength strike the surface and enter an object such as a raindrop or prism, their subsequent path depends on the original point of impact of each ray, the refractive indices of air and water and its surface properties.
- When parallel rays of incident light of a single wavelength encounter the curved surface of a transparent medium at different points, it is the differing angles at which they strike the surface and undergo deflection that are the chief determinants of the way they scatter as they exit on the other side.
About random scattering
- Random scattering results from any material that reflects or transmits rays of light in multiple directions.
- Random scattering produces reflections that appear soft and lack sharp detail.
- Light transmitted through sheets of glass with irregular but otherwise smooth surfaces distorts the appearance of the world beyond. Wherever the image becomes blurry and confused random scattering is responsible.
- A reflection that is free of the effects of random scattering is called a specular reflection.
- Objects produce diffuse reflections when light bounces off rough or uneven surfaces scattering light in all directions.
- Transparent materials transmit diffuse light due to random scattering unless their surfaces are perfectly regular (flat or curved) and their interiors are free of foreign material.
- Whilst translucent materials containing dissolved materials such as dyes do not cause random scattering, embedded impurities do.
- All objects obey the law of reflection on a microscopic level, but if the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion which causes scattering.
About scattering in raindrops
- Regular scattering takes place as light passes through raindrops as a result of refraction and chromatic dispersion.
- Refraction refers to the way that light changes speed and direction as it travels across the interface between one transparent medium and another.
- Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
- Regular scattering obeys the law of reflection and refraction (Snell’s law).
Here are three related descriptions of the way light scatters when it strikes a raindrop:
- When light of a particular wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, and the refractive indices of air and water it passes through.
- For incident rays of a single wavelength striking the surface of a raindrop at different points, it is the different angles at which they enter the droplet that is the chief determinant of the way they scatter as they exit the droplet.
- For incident rays of white light striking the surface of a single droplet at different points, it is the different angles at which they enter the droplet along with the effects of chromatic dispersion that determine the form of scattering.
Related diagrams
Each diagram below can be viewed on its own page with a full explanation.
